const isValid = function(s){
  let arr = [];
  for(let i = 0; i < s.length; i++){
    if(s[i] === '{' || s[i] === '[' || s[i] === '('){
      arr.push(s[i]);
    }else{
      if(s[i] === ')' && arr[arr.length-1] === '(') arr.pop();
      else if(s[i] === ']' && arr[arr.length-1] === '[') arr.pop();
      else if(s[i] === '}' && arr[arr.length-1] === '{') arr.pop();
      else return false;
    }
  }
  return arr.length === 0;
};